1. Field of the Invention
The present invention relates to suspension systems for off-road equipment, such as agricultural tractors, and more particularly to such suspension systems that provide hydraulic load leveling.
2. Description of the Related Art
Off-road equipment, such as the agricultural tractor 10 shown in FIG. 1, can carry widely varying loads. Often the load is an implement that is attached to a hitch 12 at the rear of the tractor. When a relatively heavy load is applied to the hitch 12, the rear of the tractor 10 is forced downward causing the tractor to pivot above the axle for the rear wheels 14. This raises the front wheels 16 which can adversely affect the maneuverability of the tractor. Similarly when a heavy load is applied to the front of the tractor, the front wheels are forced downward, which also affects steering and can affect traction of the rear wheels.
As a result, many vehicles have automatic load leveling systems which employ hydraulic cylinder between the axle and the frame of the vehicle which act as shock absorbers and which can be dynamically adjusted to maintain a proper spacing between the frame and the axle. When a heavy load is applied, the drop of the frame is sensed and additional hydraulic fluid is applied to the cylinder to raise the frame the desired distance from the axle. Thereafter, when the load is removed from the vehicle the frame will rise significantly above the axle. When this occurs hydraulic fluid is applied to the opposing chamber of the cylinder to lower the frame with respect to the axle. This type of automatic hydraulic load leveling system ensures that the frame and axle will be at the desired separation regardless of the size of the load applied to the vehicle.
One of the drawbacks of this load leveling system is that the opposite chambers of the double acting cylinder have separate pressure control circuits and require high pump pressure to move the cylinder in both directions. Thus the consumption of fluid from the pump for load leveling may adversely affect the availability of fluid pressure for other functions powered by the tractor. In order to compensate for that power consumption, the pump capacity would have to be increased thus raising the cost of the hydraulic system. Furthermore, these types of systems require relatively large accumulator volumes.
A suspension of a vehicle has a cylinder with a piston that defines a piston chamber and a rod chamber. The suspension is controlled by a hydraulic circuit that includes a first work port for connection to the rod chamber of the cylinder, and a second work port for connection to the piston chamber. A first control valve has an inlet to receive fluid from a pump on the vehicle and has an outlet coupled to the first work port. A proportional control valve selectively connects the first work port to the second work port. A pressure relief valve is connected between one of the first and second work ports and a tank return line of the vehicle, and opens in response to pressure in the respective work port exceeding a first threshold level. The first threshold level is varied by an electric signal. A first accumulator is coupled to the first work port and a second accumulator is connected to the second work port.
In the preferred embodiment, operation of the hydraulic circuit is controlled by an electronic controller that selectively operates the first control valve and the proportional control valve, and provides the electric signal to the valve pressure relief valve. The hydraulic circuit is operated in either a regeneration mode or a double acting mode depending upon pressure in the cylinder. Preferably, the regeneration mode is selected when pressure in the piston chamber exceeds a predefined level.
During normal operation of the suspension, the proportional control valve is maintained in an open state in the regeneration mode, and the proportional control valve is maintained in a closed state in the double acting mode.
That pressure is employed to determine whether to operate the suspension in a regeneration or a double acting mode. The control system making that determination is either a closed loop with pressure sensors or an open loop based on the pressure versus current characteristics of a solenoid valve in the hydraulic circuit.
The controller also operates the hydraulic circuit to raise and lower the suspension in order to compensate for changes in the load carried by the vehicle. In the regeneration mode, the suspension is lowered by adjusting the first threshold level to meter hydraulic fluid through the pressure relief valve to the tank return line, and is raised by opening the first control valve. In the double acting mode in which the rod chamber pressure is controlled, the suspension is lowered by opening the first control valve, and is raised by opening the first control valve and operating the proportional control valve to meter flow of hydraulic fluid. When the cylinder is inverted from that illustrated orientation and the rod chamber pressure is controlled in the double acting mode the operation is similar except that the first control valve is not opened to raise the vehicle.